Altaf Hussain has widespread support in Karachi but is based in Edgware
Police in London have arrested the leader of Pakistan's powerful MQM party, Altaf Hussain, on suspicion of money-laundering, his party says.
Officers are searching a residential address in north-west London, where a 60-year-old man was detained.
Mr Hussain has lived in the UK since 1991, saying his life would be at risk if he returned to Pakistan.
Police in Karachi, Pakistan's largest city, which the MQM controls, have been put on alert amid reports of violence.
The British and Pakistan authorities have in the past expressed concerns that any arrest of Altaf Hussain could lead to violent protests in Karachi.
Shootings have been reported from some parts of the city, which BBC correspondents say is tense.
Traffic jams were reported in Karachi and other cities in Sindh province as businesses closed and people headed home fearing violence.
One man in the city, who gave his name as Tahir, told the BBC that MQM supporters were firing in the streets and setting fire to any shop which remained open.
Another, Nabil, said there was "turmoil" and "massive confusion" about whether Altaf Hussain had been arrested or not.
Security is being tightened around the British mission in the city, which has been closed temporarily, and other buildings.
The city has been wracked by violence - much of it politically motivated.
Muttahida Qaumi Movement (MQM) party leaders are holding an emergency meeting at their headquarters in Karachi.
A senior MQM official said the party was not trying to conceal or hide what has happened but he added that Altaf Hussain was suffering from a medical condition and was due to be admitted to hospital on Tuesday.
Altaf Hussain, his party, his associates and relatives are currently the subject of a number of British investigations, reports the BBC's Owen Bennett-Jones.
One is looking into the question of whether in his televised speeches he has incited violence in Pakistan, charges Mr Hussain denies. Another is into whether the MQM has paid its UK taxes correctly.
The most high-profile investigation followed the 2010 murder in London of a senior MQM leader Imran Farooq. No-one has been formally charged with his killing.
Our correspondent says the MQM is often accused of extorting money from businesses in Karachi and shipping the money to the UK, charges the party also denies.
Are you in Karachi? Are you affected by issues raised in this story? Contact us at this address (please quote 'Karachi' in your heading): firstname.lastname@example.org